Project Summary Introduction: We propose to design a vaccine for Cryptosporidia by identifying antigenic targets of the parasite that are neutralized by mucosal IgA, are antigenically conserved, recognized by cell mediated immunity (CMI), and associated with protection of infants. Hypothesis: Protection from Cryptosporidia is via anti-IgA mucosal antibodies against surface-exposed antigens that provide broadly-neutralizing prevention of infection, and via cell mediated immune responses required for resolution of infection. Premise: A vaccine can be rationally designed by identifying microbial antigens that are broadly conserved and recognized by immune responses associated with protection. Significance: Cryptosporidiosis is a top 10 cause of diarrhea in the 1st year of life in low and middle-income countries. In North America it is the leading etiology of water-borne diarrhea, a biodefense category B agent, and cause of chronic diarrhea in AIDS. Investigators: The multiple PIs Drs. Petri and Gilchrist at the University of Virginia, and the foreign site lead Dr. Haque at icddr,b, have collaborated on enteric parasites for 25 years. For Cryptosporidia they have discovered in infants acquired immunity associated with a mucosal IgA response, and through genome resequencing conserved antigens. They are joined by Dr. Campo of Antigen Discovery (ADI) who brings an eminently successful track record in use of microbial proteome arrays for antigen discovery. Innovation: The identification of IgA acquired immune responses associated with protection, and application of a proteome microarray to identify targets of mucosal IgA are ground-breaking in Cryptosporidiosis Approach: There are three Specific Aims: Specific Aim 1. Assess the natural history of Cryptosporidiosis through age 5 in children in an urban slum of Dhaka, Bangladesh. Specific Aim 2: Measure the duration and magnitude of passive and active immunity associated with IgA against the parasite Cp23 antigen. Specific Aim 3: Discover antigens for a broadly neutralizing vaccine. Environment: Key to success are the complementary expertise of the team at the University of Virginia in molecular parasitology, at icddr,b in clinical investigation, and at ADI in antigen discovery. This robust collaboration has led to the substantial published and preliminary data presented in the proposal.